I am looking for an affordable folding bike with reliable warranty. So far all the bikes I looked at have 2 weeks to 3 months warranty for frame and manufacturers warranty for parts, nothing like 5 years to 10 years warranty for their fixed frame counterparts. Why are folding bike warranties so short?
Sounds like you're looking for opposing features. A good/great folding bike will cost more than the equivalent full-sized bike.
A cheap folding bike will have the bare minimum warranty permitted in your location.
For example, the warranty on a Giant bike is
Giant warrants for the original owner only the frame, rigid fork, or original component parts of each new Giant brand bicycle to be free from defects in material and workmanship for the following specified periods:
Warranty of lifetime for - The bicycle frame, except on models intended for downhill purposes.
Warranty of ten years for - Rigid forks.
Warranty of three years for - Frames on models intended for downhill purposes.
Warranty of one year for - Paint finish and decals
All other original parts, including rear shock linkages, but excluding non Giant Brand parts, suspension forks and rear shock absorbers. (All non Giant brand parts, suspension forks, and rear shock absorbers shall be covered exclusively by the stated warranty of their original manufacturer.)
Their only folder is $819 NZ or about $600 USD.
By comparison a top folder like a brompton has a 5 year warranty on the frame, and only on material and manufacturing faults. This will not cover accident damage or wear and tear.
If your bicycle has a material or manufacturing defect, we will replace the defective part free of charge if we are notified within five years (in the case of a frame part) or two years (in the case of other parts) from the date of first purchase of the bicycle.
Their folders range from £800 to £2200, so $1100 to $3000 USD.
In short, you get the warranty you pay for.
Folders are more likely to go wrong than full sized frames, particularly due to the hinge that allows the bike to fold (this can become loose/damaged). Since folders are more complicated than regular frames and a niche product, you have to pay more to get a folder of equivalent quality than a regular frame and it doesn't really make sense to warranty something thats more fragile for as long.
That being said, some manufacturers do offer 5 year warranties -- Dahon, for example. But you do have to have periodic maintenance records and what not.
In any case, regardless of what the warranty says, its more important to have a manufacturer who builds solid bicycles. Standing behind the warranty is another good trait -- Having a 10000 year warranty is useless if you can't actually use it in the case of a defect. And if something goes wrong that isn't a defect (like you were riding around with the hinge not being tight or something), note that the warranty won't cover you. Also, remember things like lifetime warranties are for the bike's useful expected life, not yours.